An arm slung defensively across his chest, shoulders sagging, Councillor James Powney gave a presentation entitled Transforming Brent's Libraries.

Explaining why the north-west London council was closing six out of 12 was the unenviable job that faced him in a cavernous town hall conference room this week. "It's the easy option – who cares about books?" someone shouted. "This is our heritage, this is our children's future," yelled another woman.

Under a barrage of heckling, Powney said: "If you want a library to stay open, it will not come from beating me up. There is no point in mugging someone with no money." The shouting did not subside.

Brent is one of hundreds of councils planning to close public libraries in an attempt to meet huge cuts imposed by central government. Nearly 400 are threatened with closure, and with half of councils yet to announce their plans, the final number could be as many as 800 – a fifth of all libraries.

One of those earmarked is Kensal Rise library. Housed in a comfortingly solid Victorian building, near a primary school on a quiet residential street, it was opened in 1900 by Mark Twain.

Caroline Bottomley, 49, has been going there for 12 years. "I walk down the road on my way home from the tube, and when I see those brass chandeliers twinkling in the windows, it feels like a homecoming," she said. "It's not just a library, it's such a big symbol of the community."

It is homely, with dark wood parquet floors, old-fashioned bookshelves, hushed voices. Students ploughing through their books, an older woman slowly turning pages of the Daily Mail.

Sara Levy, 39, sat with three-year-old Ella, colouring in Spot the Dog on a computer screen, with three-week-old Ava asleep on her chest. "It's the only local place you can come without spending money," she said. "It's local and friendly. And it's classless."

It is a despair shared throughout the country. Campaigners say such severe measures go too far, given that council funding cuts have been capped at 8.9% this year, and libraries are being seen as a soft touch by councils struggling to meet other legally binding commitments.

Ann John, Labour leader of Brent council, disputes this. "Like any administration, we would not willingly close a library because politically it is too difficult," she said. Legally, it is difficult too. Under the 1964 Museums and Public Libraries Act authorities must provide a "comprehensive and efficient" service to all. But with £37m to cut from its budget this year, the council has little choice, she argues. "We have never seen reductions on this scale before."

Libraries were already under siege before the recession struck. Although the number has stayed largely stable over the past decade at the 4,500 mark, there has been a relentless decline in library use. Some argue fewer people visit libraries because the core service – book lending – has been badly run.

Many campaigners warn that councils are choosing to keep larger urban libraries but scrap local branches relied on by those unable to travel easily – the elderly, the poor, parents with young children. More than 300m book loans were made from public libraries last year, according to the Chartered Institute of Public Finance and Accountancy, yet there are libraries under threat in every part of the country.

Doncaster is set to lose 14 of its 26 libraries, with poorer areas most affected, said Joyce Sheppard of the Save Doncaster Libraries campaign. "Doncaster is such a poor area, with low attainment in literacy, and these cuts matter," she said.

In Somerset the council is proposing to close 20 out of 34 libraries. Conwy, north Wales, is aiming to close seven of 12. Croydon, south London, is to lose five. A day of action on 5 February will see "read-ins" at many of the threatened libraries in areas including Lewisham, south-east London, and North Yorkshire, with local authors joining readers, librarians and councillors to show support in quiet, family-friendly demonstrations. Others are planning flashmobs, YouTube videos, and picketing.

Last year when Labour considered changes that would make closing libraries easier Ed Vaizey – then shadow culture minister and now the man in charge – made an impassioned defence of libraries. "This puts the future of every public library under threat. This is outrageous and offensive to everyone who ever cared about books and reading," he roared.

Now, in his own backyard – like David Cameron, he represents an Oxfordshire constituency – the council is cutting 20 of its 43 libraries. In the spirit of the "big society", local communities will be offered the chance to run them instead, but this would be an "absolute disaster", according to children's author and Oxfordshire resident Mary Hoffman. "Libraries need qualified librarians, and I don't see volunteers rushing forward in some of the areas Oxfordshire has chosen to target. It's vandalism."